This invention relates to data communication apparatus and, more particularly, to apparatus for digitally generating bandpass signals.
Data transmission over voice frequency channels is generally accomplished by modulating a baseband data signal onto a carrier signal, developing thereby a bandpass signal, and then transmitting the modulated signal to a distant receiver wherein the signal is demodulated. The transmitter's modulation process generally includes low pass filtering to insure that the baseband signal is of a finite known bandwidth, modulating onto a carrier by means of any of a variety of modulation schemes, and bandpass filtering to insure that the transmitted signal occupies a finite known frequency band.
With the present trend of semiconductor technology toward large scale integration, a number of attempts have been made to implement the transmitter functions by digital techniques in a fashion that lends itself to integration. In an article entitled "In-Band Generation of Synchronous Linear Data Signals," IEEE Trans. on Comm., Vol. COM 21 No. 10, Oct. 1973, page 1116, Kalet and Weinstein describe a bandpass signal generator employing a finite number of fixed filters which, in combination, develop bandpass signals. In an article entitled, "Microcoded Modem Transmitters," IBM J. Res. Develop., July 1974, pp. 338-351, Choquet et al described a transmitter design employing a digital finite impulse response filter and programmable, cyclically modified, filter coefficients to obtain the desired bandpass signal. Both the Kalet and the Choquet apparatus are useful only when the ratio of the desired carrier frequency to the filters' processing rate is a rational number.